This invention relates to radioactivity well logging, and, more particularly, to novel methods and apparatus for identifying the density and lithology of subsurface earth formations.
It has been known that gamma rays of a known energy spectrum, after introduction into a subsurface earth formation, will undergo mechanisms discussed in the literature such as absorptions due to the photoelectric effect and Compton scattering. These mechanisms will alter the energy spectrum of gamma rays exiting the formation as a function of formation characteristics. Thus, very valuable information regarding formation parameters such as density, lithology, and the like, are contained in the resultant spectrum.
More particularly, it has been demonstrated that the ratios of count rates of such exiting gamms rays for energies about the (50-100 Kev)/(200-540 Kev) range exhibit a strong functional relationship to the photoelectric cross-section P.sub.e and thus the lithology of the formation. In like manner, it has further been known that the count rate of such gamma rays in higher energy ranges of, say, 200-540 Kev, exhibit a like strong functional dependence upon formation density, .rho..
Accordingly, it has been conventional to provide a logging tool having a source of gamma rays of a known spectrum and intensity which are introduced into the formation, and a means for detecting gamma rays incident upon the tool and originating from the formation. Representative such systems and a discussion of their general theory of operation may be seen depicted in U.S. Pat. No. 3,864,569 to Tittman; U.S. Pat. No. 3,858,037 to Moore, et al; and U.S. Pat. No. 3,321,625 to Wahl.
Due to the aforementioned significance of the relative energy levels of the detected gamma rays, in these systems it is conventional to provide means for developing a count rate of gamma rays having energies within preselected energy bands. Accordingly, electronic circuit means are provided in the tool known as energy window discriminators through which only pulses of a preselected voltage amplitude range are permitted to pass for counting. Since these pulses correspond in amplitude to the detected gamma ray energies from a gamma ray detector, the desired count rate of gamma rays within a preselected energy band of interest is thus made available.
However, several serious disadvantages are present in the hereinabove described systems. First, the discriminators are typically comprised of analog circuitry particularly sensitive to adverse borehole conditions of extreme temperature, vibration, and the like. These circuits have been known to drift such that a count rate from such a discriminator thought to correspond to one energy range might, in reality, correspond to that of gamma rays of a different range.
Moreover, each discriminator is conventionally comprised of a separate circuit. Thus the number of discriminators and correlative energy bands for which count rates may be provided is limited by the size constraints of logging tools. Accordingly, such count rates are typically only provided for two energy bands. The first band corresponds to "soft" or relatively low energy level gamma rays typically within the range of 60-100 Kev wherein primarily lithology information is contained. The other band corresponds to "hard" or relatively high energy level gamma rays approximately within the range of 200-550 Kev wherein the functional relationship between gamma ray energies detected in the formation is strongly dependent upon formation densities.
Still further, not only is the resolution of the count rates in corresponding energy bands limited due to the aforementioned borehole conditions and limited number of discriminators available. Due to the "hard wired" nature of the downhole discriminator circuitry there is little if any opportunity to alter the windows in terms of their number, energy range, and the like.
Due to the hereinbefore mentioned limitations of prior apparatus (including the limited number of window discriminators and their inherent instability), it was further difficult to locate with desired precision in the data transmitted uphole an energy calibration point, such as a 660 Kev energy peak from an internal monitor cesium source, which would permit translation of count rates to actual energy spectra. Moreover, no effective way was provided to check gamma ray detector resolution so as to check discriminator window accuracies and to compensate for resolution effects on observed spectra, in that due to the limited spectral resolution of the conventional windows, accurate determination of the monitor peak width and energy was not possible.
Still further, due to the limited number of energy windows and associated count rates, statistical errors larger than desired resulted in density determinations. Moreover, errors due to the effect of the photoelectric cross-section P.sub.e on density could not be minimized. Also, the effect of density on the soft-to-hard ratio was not compensated for by consideration of density in determination of P.sub.e. These and other disadvantages of the prior art are overcome by the well logging method and apparatus of the present invention.